Here is the key thing you need to understand about grapplers: they are not killing machines, they are denial
specalists. A well built grappler can choose most any other humanoid his own size or smaller and remove that opponent from the fight. The cost? The grappler also removes himself from the fight. By now you should be starting to see the pardigm. A good grappler should try to pick the highest priority target and nutralize him, he then has to wait for the rest of his team to either wipe out the opposition or else come and help him beat up the opponent he is grappling. If the grappler chooses the wrong opponent to nutralize or his team fails him then he is left with his ass hanging out in the breeze.
So playing a good grappler is a very tactical position. It is a very powerful build for two reasons. 1) you are forcing
the defender to play on your terms. The defender has
to beat you in an opposed grapple check and until he does he is unable to do much of anything but try to escape the grapple. 2) since few characters take Imp Grapple merely as a defense to grapplers it means you will probably have an advantage over other human grapplers; and if the defender doesn't have a full BAB then your victory is almost guaranteed. IOW you can basically walk up to that big barbarian who is doing so much damage with his greatsword, grapple him, and now he is virtually useless since he has to try and fight you with light weapons or unarmed attacks.
What are the weaknesses of grapplers? Well if the defender has any friends in the area, espically any friends with sneak attack dice, then the grappler is asking to have his throat cut. Second he does very little damage per round. It takes forever to kill someone in a grapple so the grappler really does need to rely on the rest of his party to either gang up on the defender with him or to face the rest of the opposition without him. Lastly grappling is mostly useless against anyone who is a larger size than you as well as many non-humanoid opponents, you do not want to grapple most monsters.
As for how to build a good grappler, you need
A full BAB (barbarian or soldier would work best)
A very good Str score
Improved Unarmed Strike
You also want a good Con and toughness because you will probably wind up taking lots of small hits over time. Stunning Attack is also good with grapples.
Weapon Focus/Specialisation is good though please note that by the RAW Weapon Focus "unarmed strike" and Weapon Focus "grapple" are actually two seperate feats
which really hurts grapplers. IMC I ignore this as a house rule
As for the feat Crushing Grip, I think it is way overpowered and as a house rule
I have banned it. I replaced it with the feat Earth's Embrace from OA.
First, grapples deal 1d4+str damage as unarmed strikes. So taking brawl will increase that to 1d6, and let you take Improved unarmed strike once for 1d8 and up to 1d12 if you are from an oriental land.
What, no. No, you deal 1d6 + Str with the Brawl feat and that is it. I think you have confused Imp Unarmed strike with Improved Natural Attack, which is a completly different feat and not in the Conan rules.
Hope that helps.